There's nothing wrong with your camper. The converter will always draw some power, even when the batteries are full. They are always putting a bit of a charge in there, particularly the older ones that weren't very "smart".
The little red light battery gauge is basically worthless. There is no reason to even look at it. It goes by voltage only so when plugged in with the converter running it will always read full, even if your batteries are shot and worthless.
Not long after unplugging the converter it will drop to 2/3 just because the surface charge on the batteries has dissipated. Worthless.